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Why is the Mona Lisa the most famous painting in the world? Why did Facebook succeed when other social networking sites failed? Did the surge in Iraq really lead to less violence? How much can CEO's impact the performance of their companies? And does higher pay incentivize people to work hard? If you think the answers to these questions are a matter of common sense, think again. As sociologist and network science pioneer Duncan Watts explains in this provocative book, the explanations that we give for the outcomes that we observe in life---explanation that seem obvious once we know the answer---are less useful than they seem. Drawing on the latest scientific research, along with a wealth of historical and contemporary examples, Watts shows how common sense reasoning and history conspire to mislead us into believing that we understand more about the world of human behavior than we do; and in turn, why attempts to predict, manage, or manipulate social and economic systems so often go awry. It seems obvious, for example, that people respond to incentives; yet policy makers and managers alike frequently fail to anticipate how people will respond to the incentives they create. Social trends often seem to have been driven by certain influential people; yet marketers have been unable to identify these "influencers" in advance. And although successful products or companies always seem in retrospect to have succeeded because of their unique qualities, predicting the qualities of the next hit product or hot company is notoriously difficult even for experienced professionals. Only by understanding how and when common sense fails, Watts argues, can we improve how we plan for the future, as well as understand the present---an argument that has important implications in politics, business, and marketing, as well as in science and everyday life..

CHAPTER 1

The Myth of Common Sense

Every day in New York City five million people ride the subways. Starting from their homes throughout the boroughs of Manhattan, Brooklyn, Queens, and the Bronx, they pour themselves in through hundreds of stations, pack themselves into thousands of cars that barrel though the dark labyrinth of the Metropolitan Transportation Authority's tunnel system, and then once again flood the platforms and stairwells-a subterranean river of humanity urgently seeking the nearest exit and the open air beyond. As anyone who has ever participated in this daily ritual can attest, the New York subway system is something between a miracle and nightmare, a Rube Goldberg contraption of machines, concrete, and people that in spite of innumerable breakdowns, inexplicable delays, and indecipherable public announcements, more or less gets everyone where they're going, but not without exacting a certain amount of wear and tear on their psyche. Rush hour in particular verges on a citywide mosh pit-of tired workers, frazzled mothers, and shouting, shoving teenagers, all scrabbling over finite increments of space, time, and oxygen. It's not the kind of place you go in search of the milk of human kindness. It's not the kind of place where you'd expect a perfectly healthy, physically able young man to walk up to you and ask you for your seat.

And yet that's precisely what happened one day in the early 1970s when a group of psychology students went out into the subway system on the suggestion of their teacher, the social psychologist Stanley Milgram. Milgram was already famous for his controversial "obedience" studies, conducted some years earlier at Yale, in which he had shown that ordinary people brought into a lab would apply what they thought were deadly electrical shocks to a human subject (really an actor who was pretending to be shocked) simply because they were told to do so by a white-coated researcher who claimed to be running an experiment on learning. The finding that otherwise respectable citizens could, under relatively unexceptional circumstances, perform what seemed like morally incomprehensible acts was deeply disturbing to many people-and the phrase "obedience to authority" has carried a negative connotation ever since.1

What people appreciated less, however, is that following the instructions of authority figures is, as a general rule, indispensible to the proper functioning of society. Imagine if students argued with their teachers, workers challenged their bosses, and drivers ignored traffic cops anytime they asked them to do something they didn't like. The world would descend into chaos in about five minutes. Clearly there are moments when it's appropriate to resist authority, and most people would agree that the situation Milgram created in the lab would qualify as such a moment. But what the experiment also illustrated was that the social order that we take for granted in everyday life is maintained in part by hidden rules that we don't even realize exist until we try to break them.

Based on this experience, and having subsequently moved to New York, Milgram had begun to wonder if there was a similar "rule" about asking people for seats on the subway. Like the rule about obeying authority figures, this rule is never really articulated, nor would a typical rider be likely to mention it if asked to describe the rules of subway riding. And yet it exists, as Milgram's students quickly discovered when they went about their little field experiment. Although more than half of the riders asked eventually surrendered their seats, many of them reacted angrily or demanded some explanation for the request. Everyone reacted with surprise, even amazement, and onlookers often made disparaging remarks. But more interesting than the response of the riders was that of the experimenters themselves, who found it extremely difficult to perform the experiment in the first place. Their reluctance was so great, in fact, that they had to go out in pairs, with one of them acting as moral support for the other. When the students reported their discomfort to Milgram, he scoffed at them. But when he tried to do the experiment himself, the simple act of walking up to a complete stranger and asking for his or her seat left him feeling physically nauseated. As trivial as it seemed, in other words, this rule was no more easily violated than the obedience-to-authority "rule" that Milgram had exposed years earlier.2

As it turns out, a big city like New York is full of these sorts of rules. On a crowded train, for example, it's no big deal if you're squeezed in against other people. But if someone stands right next to you when the train is empty, it's actually kind of repellant. Whether it's acknowledged or not, there's clearly some rule that encourages us to spread out as much as we can in the available space, and violations of the rule can generate extreme discomfort. In the same way, imagine how uncomfortable you'd feel if someone got on your elevator and stood facing you instead of turning around to face the door. People face each other all the time in enclosed spaces, including on subway trains, and nobody thinks twice about it. But on an elevator it would feel completely weird, just as if the other person had violated some rule-even though it might not have occurred to you until that moment that any such rule existed. Or how about all the rules we follow for passing one another on the sidewalk, holding open doors, getting in line at the deli, acknowledging someone else's right to a cab, making just the right amount of eye contact with drivers as you cross the street at a busy intersection, and generally being considerate of our fellow human beings while still asserting our own right to take up a certain amount of space and time?

No matter where we live, our lives are guided and shaped by unwritten rules-so many of them, in fact, that we couldn't write them all down if we tried. Nevertheless, we expect reasonable people to know them all. Complicating matters, we also expect reasonable people to know which of the many rules that have been written down are OK to ignore. When I graduated from high school, for example, I joined the Navy and spent the next four years completing my officer training at the Australian Defence Force Academy. The academy back then was an intense place, replete with barking drill instructors, predawn push-ups, running around in the pouring rain with rifles, and of course lots and lots of rules. At first this new life seemed bizarrely complicated and confusing. However, we quickly learned that although some of the rules were important, to be ignored at your peril, many were enforced with something like a wink and a nod. Not that the punishments couldn't be severe. You could easily get sentenced to seven days of marching around a parade ground for some minor infraction like being late to a meeting or having a wrinkled bedcover. But what you were supposed to understand (although of course you weren't supposed to admit that you understood it) was that life at the academy was more

like a game than real life. Sometimes you won, and sometimes you lost, and that was when you ended up on the drill square; but whatever happened, you weren't supposed to take it personally. And sure enough, after about six months of acclimation, situations that would have terrified us on our arrival seemed entirely natural-it was now the rest of the world that seemed odd.

We've all had experiences like this. Maybe not quite as extreme as a military academy-which, twenty years later, sometimes strikes me as having happened in another life. But whether it's learning to fit in at a new school, or learning the ropes in a new job, or learning to live in a foreign country, we've all had to learn to negotiate new environments that at first seem strange and intimidating and filled with rules that we don't understand but eventually become familiar. Very often the formal rules-the ones that are written down-are less important than the informal rules, which just like the rule about subway seats may not even be articulated until we break them. Conversely, rules that we do know about may not be enforced, or may be enforced only sometimes depending on some other rule that we don't know about. When you think about how complex these games of life can be, it seems kind of amazing that we're capable of playing them at all. Yet, in the way that young children learn a new language seemingly by osmosis, we learn to navigate even the most novel social environments more or less without even knowing that we're doing it.

COMMON SENSE

The miraculous piece of human intelligence that enables us to solve these problems is what we call common sense. Common sense is so ordinary that we tend to notice it only when it's missing, but it is absolutely essential to functioning in everyday life. Common sense is how we know what to wear when we go to work in the morning, how to behave on the street or the subway, and how to maintain harmonious relationships with our friends and coworkers. It tells us when to obey the rules, when to quietly ignore them, and when to stand up and challenge the rules themselves. It is the essence of social intelligence, and is also deeply embedded in our legal system, in political philosophy, and in professional training.

For something we refer to so often, however, common sense is surprisingly hard to pin down.3 Roughly speaking, it is the loosely organized set of facts, observations, experiences, insights, and pieces of received wisdom that each of us accumulates over a lifetime, in the course of encountering, dealing with, and learning from, everyday situations. Beyond that, however, it tends to resist easy classification. Some commonsense knowledge is very general in nature-what the American anthropologist Clifford Geertz called an "ancient tangle of received practices, accepted beliefs, habitual judgments, and untaught emotions."4 But common sense can also refer to more specialized knowledge, as with the everyday working knowledge of a professional, such as a doctor, a lawyer, or an engineer, that develops over years of training and experience. In his address to the annual meeting of the American Sociological Society in Chicago in 1946, Carl Taylor, then president of the association, put it as well as anyone:

By common sense I mean the knowledge possessed by those who live in the midst and are a part of the social situations and processes which sociologists seek to understand. The term thus used may be synonymous with folk knowledge, or it may be the knowledge possessed by engineers, by the practical politicians, by those who gather and publish news, or by others who handle or work with and must interpret and predict the behavior or persons and groups.5

Taylor's definition highlights two defining features of common sense that seem to differentiate it from other kinds of human knowledge, like science or mathematics. The first of these features is that unlike formal systems of knowledge, which are fundamentally theoretical, common sense is overwhelmingly practical, meaning that it is more concerned with providing answers to questions than in worrying about how it came by the answers. From the perspective of common sense, it is good enough to know that something is true, or that it is the way of things. One does not need to know why in order to benefit from the knowledge, and arguably one is better off not worrying about it too much. In contrast with theoretical knowledge, in other words, common sense does not reflect on the world, but instead attempts to deal with it simply "as it is."6

The second feature that differentiates common sense from formal knowledge is that while the power of formal systems resides in their ability to organize their specific findings into logical categories described by general principles, the power of common sense lies in its ability to deal with every concrete situation on its own terms. For example, it is a matter of common sense that what we wear or do or say in front of our boss will be different from how we behave in front of our friends, our parents, our parents' friends, or our friends' parents. But whereas a formal system of knowledge would try to derive the appropriate behavior in all these situations from a single, more general "law," common sense just "knows" what the appropriate thing to do is in any particular situation, without knowing how it knows it.7 It is largely for this reason, in fact, that commonsense knowledge has proven so hard to replicate in computers-because, in contrast with theoretical knowledge, it requires a relatively large number of rules to deal with even a small number of special cases. Let's say, for example, that you wanted to program a robot to navigate the subway. It seems like a relatively simple task. But as you would quickly discover, even a single component of this task such as the "rule" against asking for another person's subway seat turns out to depend on a complex variety of other rules-about seating arrangements on subways in particular, about polite behavior in public in general, about life in crowded cities, and about general-purpose norms of courteousness, sharing, fairness, and ownership-that at first glance seem to have little to do with the rule in question.

Attempts to formalize commonsense knowledge have all encountered versions of this problem-that in order to teach a robot to imitate even a limited range of human behavior, you would have to, in a sense, teach it everything about the world. Short of that, the endless subtle distinctions between the things that matter, the things that are supposed to matter but don't, and the things that may or may not matter depending on other things, would always eventually trip up even the most sophisticated robot. As soon as it encountered a situation that was slightly different from those you had programmed it to handle, it would have no idea how to behave. It would stick out like a sore thumb. It would always be screwing up.8

People who lack common sense are a bit like the hapless robot in that they never seem to understand what it is that they should be paying attention to, and they never seem to understand what it is that they don't understand. And for exactly the same reason that programming robots is hard, it's surprisingly hard to explain to someone lacking in common sense what it is that they're doing wrong. You can take them back through various examples of when they said or did the wrong thing, and perhaps they'll be able to avoid making exactly those errors again. But as soon as anything is different, they're effectively back to square one. We had a few cadets like that at the academy: otherwise perfectly intelligent, competent people who just couldn't seem to figure out how to play the game.